The present disclosure is directed to a method and apparatus for correcting the MWD porosity for standoff between the tool and the sidewall of the borehole. This is particularly intended for use with a tool which is constructed in a drill collar equipped with a lengthwise stabilizer fin. The stabilizer fin is provided with an ultrasonic measuring signal which transmits a signal radially outwardly which is reflected back to the transducer of the ultrasonic device so that a measurement of spacing can be obtained. The sidewall of the borehole is normally represented as an idealized circular surface; in reality, it is not circular but is an irregular surface which varies irregularly in spacing from the drill collar which supports the MWD tool. The stabilizer fin can either be helical or straight along one side of the drill collar; indeed, many drill collars are made with two or three stabilizer fins in helical form extending around and along the drill collar. The ultrasonic standoff detector measures spacing between the stabilizer fin and the adjacent wall of the borehole so that standoff can then be determined.
Porosity is ordinarily measured by positioning in the stabilizer fin some type of radiation source and a pair of spaced detectors responsive to the source. The source cooperates with the two detectors which provide a detected count rate at each of the two detectors. The count rate is normally dealt with by determining a ratio between the counts from the near and far detectors, and this ratio is normally represented as the ratio of N/F. The N/F ratio is a relative value and hence cancels from the numerator and denominator equally any variations which might arise from changes in source intensity or other scale values which might cause variations in absolute measurements. This is desirable so that the value of the N/F ratio can be correlated to a porosity measurement for a particular formation adjacent to the well borehole. The correlation between the ration N/F and the porosity is determined from measurements made in standard calibration facilities with no standoff. Deviations from the true porosity occur when the standoff is not zero. If the standoff is not zero the apparent porosity can be corrected to obtain a measure of the true porosity may be nonlinear.
The context in which the MWD equipment is used must also be noted. That is, the MWD equipment described herein is mounted in a drill collar which is rotating at the time that measurements are taken. In light of the fact that the tool is rotating and the hole is not perfectly round, the standoff may fluctuate radically several times during one revolution. The rate of change can be quite high and is irregular in nature. Moreover, a simple average value of standoff cannot be used to obtain a correct measurement of porosity because the correction based on standoff may not be linear. The present invention sets forth both a method and apparatus by which the standoff is measured repetitively during rotation and different values are obtained for such measurements. In fact, the standoff measurements are used to steer pulse counts occurring at that interval into specified detector registers or counters. Recall that the porosity is normally determined by irradiating the adjacent formation from the radioactive source and detecting responsive counts at both gamma ray detectors. The counts are thus stored in different counters; similar replicated sets of counters are provided for the counts from both the near and far detectors. The counts are thus stored in their respective counters, and the two sets of counters are then matched to obtain the N/F ratio for each of the respective counters in the two sets. For instance, if there are eight near counters, there should likewise be eight far counters; the near counters as well as the far counters are designated in relation to the particular standoff distance when the counts occur. This enables several different ratios to be obtained but they are more true in light of the fact that standoff matching does occur, and with this, the several counters provide several ratios. This then yields several values of porosity and these values may be averaged to provide porosity of the formation. This avoids error arising from the nonlinear relationship between the N/F ratio and standoff distance.
In the preferred embodiment, the present structure utilizes a standoff sensor which measures the distance from the MWD porosity measuring equipment to the sidewall, and provides a signal indicative of spacing. As spacing is varied, counts occurring at that spacing are steered to different counters. Preferably, the near detector as well as the far detector are both connected to equal sets of counters; both sets preferably are equal so that two sets have n counters each (where n is a whole number integer) and that in turn enables the formation of n ratios (N/F) which each are then corrected to provide a weighted average porosity.
It should be noted here that this method is applicable also if the commonly-used technique of depth shifting is used in the processing. This technique involves combining the far detector count rate, obtained with the tool at one depth, with the near detector count rate, obtained with the tool at a greater depth, to form the ratio N/F. Depth shifting is used to eliminate anomalously large porosity estimates near stratigraphic bed boundaries. The standoff correction method disclosed herein can be used along with depth shifting if count rates are recorded and stored as a function of standoff for use with count rates recorded as a function of standoff during a subsequent counting period. The ratio N/F is then formed by combining the far detector count rate corresponding to a given standoff with the near detector count rate corresponding to the same standoff distance, but from a previous counting period.